If you think that 3D shooters are boring and have Linux running, check this out:http://www.mushware.com/A free (GPL) version is available for Linux, and is (at least in Ubuntu 8.04) available via the package management (search for Adanaxis).

It's a space shooter where you control a spacecraft that is able to move in 4 dimensions, and you also have to aim in 4D to fire at the enemy ships. Cool